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atio. Given this rate of destruction, one could irradiate the actinides for several years and then store
the discharge in a geological repository where further natural decay would take place. So, if one
desires to destroy certain minor actinides, there is merit in using fast neutrons after plutonium is
transmuted in a thermal spectrum. And if one wishes to do so, it helps that the amount of actinides is
a very small portion of the initial waste (0.1%) since the fast transmutation assemblies end up being
smaller than they would have to be to transmute plutonium as well. These findings form the basis for
a transmutation scheme utilising both fast and thermal neutron spectra as discussed in the next
section.
4. Burn-up possibilities
Based on the above considerations, we have explored the process of using (1) thermal neutrons
(near the cross-section resonance peak) to do what they do best, i.e. fission plutonium, and (2) fast
neutrons to fission minor actinides. One advantage of using this process is that most of the
transmutation of plutonium is done in the thermal regime where technologies are more mature and
development risks are lower. Since the amounts of minor actinides found in the waste material are
significantly lower than the amounts of plutonium, the fast assemblies needed can be significantly
smaller or fewer than the thermal assemblies.
The fuel cycle that we have studied for this scheme is as described in the previous section: three
years of transmutation of plutonium and minor actinides in a thermal neutron spectrum assembly
operating in the critical mode, followed, without reprocessing, by one year of transmutation in the
same thermal neutron spectrum assembly operating as an accelerator-assisted sub-critical system. At
this point, essentially all fissionable materials are burned up. What remains is mainly non-fissionable
minor actinides, which are moved to a fast neutron spectrum assembly operating as a sub-critical
system.
Burn-up calculation results for this fuel cycle are shown in Figure 4 for an initial 1 000 kg charge
of weapons-grade plutonium. As the figure indicates, most of the Pu transmutation is accomplished in
a thermal critical regime. When this is followed by a one-year irradiation step in a thermal sub-critical
regime (accelerator driven), essentially all 239 Pu is gone. At this time, a three-year step of
transmutation in a fast subcritical regime leads to Point C in the chart, when most of the initial charge
is gone. If this remaining material is placed in a repository for 200 years, only 60 out of 1 000 kg of
the initial charge are left.
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